A variety of techniques are used to make high density circuit modules. Some techniques require special circuit board designs, while other techniques use conventional circuit boards.
Memory expansion is one of the many fields in which high density circuit board solutions provide space-saving advantages. For example, the well-known DIMM (Dual In-line Memory Module) board has been used for years, in various forms, to provide memory expansion. A typical DIMM includes a conventional PCB (Printed Circuit Board) with memory devices and supporting digital logic devices mounted on both sides. The DIMM is typically mounted in the host computer system by inserting a contact-bearing edge of the DIMM into a card edge connector. Typically, systems that employ DIMMs provide limited space for such devices and most memory expansion boards are somewhat limited in the memory capacity they add to a system.
There are several known methods to improve the limited capacity of a DIMM or other circuit board. Such methods have various cost or performance impacts. Further, many capacity increasing techniques exacerbate profile issues and contribute to thermal management complexities.
In one scheme, small circuit boards (daughter cards) are connected to the DIMM to provide extra mounting space, The additional connection may cause however, flawed signal integrity for the data signals passing from the DIMM to the daughter card. For example, signal traces between devices on the DIMM and devices on the daughter card may at higher speeds add to signal dispersion while added connectors are a considerable reliability issue. Other problems may arise from the connector that attaches the daughter card to the DIMM. Such flaws may cause reflections and compromise the quality of signaling waveforms and reduce the maximum speed at which the devices may operate.
Another scheme to increase circuit board capacity is multiple die packages (MDP). This scheme increases the capacity of the memory devices on the DIMM by including multiple semiconductor die in a single device package. The additional heat generated by the multiple die typically requires, however, additional cooling capabilities to operate at maximum operating speed. Further, the MDP scheme may exhibit increased costs because of increased yield loss from packaging together multiple die that are not fully pre-tested.
Yet another strategy to increase circuit board capacity is stacked packages. This scheme increases capacity by stacking packaged integrated circuits to create a high-density circuit module for mounting on the circuit board. In some techniques, flexible conductors are used to selectively interconnect packaged integrated circuits. Staktek Group L.P. has developed numerous systems for aggregating CSP (chipscale packaged) devices in space saving topologies. The increased component height of some stacking techniques may alter, however, system requirements such as, for example, required cooling airflow or the minimum spacing around a circuit board on its host system.
Typically, the known methods raise thermal management issues. For example, when a conventional FBGA packaged DRAM is mounted on a DIMM, the primary thermal path is through the balls into the core of a multilayer DIMM. When, for example, a stack of devices is employed on a DIMM, the top device gets hotter when it is active versus when the lower device is active, thus stacking methods in DIMM applications may present thermal constraints.
What is needed therefore are methods and structures for providing high capacity circuit boards in thermally efficient, reliable designs that perform well at higher frequencies but are not too large, yet can be made at reasonable cost with commonly available and readily managed materials.